The overall objective of these studies is to elucidate factors involved in maintaining normal T cell differentiation in the thymus and their decline with age. Three hypotheses will be tested: 1) that disturbance of thymocyte subsets, such as CD4 depletion after HIV infection, negatively affects differentiation of T cell precursors in the thymus; 2) changes in cell-cell interactions in the thymus and response to cytokines and chemokines affect recruitment and differentiation of T cell precursors in normal children and 3) that efficient T cell regeneration is dependent on the age of the thymus. Aim 1: To mimic the depletion of CD4+ thymocytes observed in HIV infection and to study the potential regulatory feedback of this depletion, the investigators will deplete CD4+ cells (or not deplete CD4+ cells) by adding anti-CD4 to thymic organ culture medium or by injecting it into the SCID-hu mouse. They will then compare the maturation of fluorescent-labeled CD34+ cell obtained from cord blood or other sources and added to either system. They postulate that the presence of mature CD8+ cells will impair or prevent maturation of T cell precursors. They will also study the effects of other subsets in a similar manner, such as CD3+CD45RA+, CD27+ or CD69+ populations by using antibodies to a particular subset or by irradiation and then inject precursors alone or precursors in the presence of purified thymocyte subsets from the same thymus. In addition, they will study the effect of removal of subsets in thymocyte suspension culture in the presence of cytokines. To study the effects of cytokines and chemokines on T cell development, they will examine (in thymic organ culture and in the SCID-hu mouse) the effect of a CXCR4 antagonist to determine if interaction between CXCR4 and its ligand SDF is essential for development of T cell precursors. They will use a chemotaxis assay to study precursor recruitment to the thymus.